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LESSON 13

Glossary: Acids and Bases

Acid

(Arrhenius definition) any substance that, when

dissolved in water, increases the concentration of

hydronium ion, H30' (hydrogen ion, H'). (Bronsted-Lowry

definition) the species (molecule or ion) that donates a

proton to another species in a proton-transfer reaction.

Amphiprotic

species a species that can act either as an acid or as a base

Anion a negatively charged ion

Base

(Arrhenius definition) any substance that, when

dissolved in water, increases the concentration of

hydroxide ion, OH-. (Bronsted-Lowry definition) The

species (molecule or ion) that accepts a proton in a

protontransfer reaction

Bronsted-

Lowry concept

a concept of acids and bases in which an acid is the

species donating a proton in a protontransfer reaction,

whereas a base is the species accepting a proton in such

a reaction

Cation a positively charged ion

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Hydronium

ion

the H30' ion; also called the hydrogen ion and written

H'(aq)

Ionic equation

a chemical equation for a reaction involving ions in

solution in which soluble substances (ones that dissolve

readily) are represented by the formulas of the

predominant species in that solution

pH the negative of the logarithm of the molar

hydronium-ion concentration

Spectator ions ions in an ionic equation that do not take part in the

reaction

Helpful link: Visit the link below to view a video on the properties of acids and bases.

http://www.khanacademy.org/video/acid-base-introduction

http://www.backpack.tv/video/chemistry/bozeman-science/paul-andersen/acids-bases-and-ph

Helpful link: Visit the link below to view a video on the strengths of acids and bases.

http://www.khanacademy.org/video/ph--poh-of-strong-acids-and-bases

Typically, we recognize acids and bases by their simple properties, such as taste. We know that a lemon is sour, so it is acidic. Bases tend to taste bitter. Acids and bases also change the color of certain dyes, such as phenolphthalein and litmus. Acids change litmus treated paper from blue to red. Acids change basic phenolpthalein from red to colorless.

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Bases change litmus treated paper from red to blue and phenolphthalein from colorless to pink. Acids and bases neutralize the action of each other. This is why we take antacids for stomachaches, because the antacid is a base, and neutralizes the acid in the stomach.

Arrhenius Concept of Acids and Bases

This guy named Arrhenius concocted the first successful concept of acids and bases. He did this by defining acids and bases according to the effect these substances have on water. The Arrhenius concept of acids and bases is as follows: an acid is a substance that when dissolved in water increases the concentration of the hydrogen ion, H+. A base is a substance that when dissolved in water increases the concentration of the hydroxide ion, OH-.

The hydrogen ion, is not just a bare proton, it is a proton bonded to a water molecule, H2O. This results in a hydronium ion, H3O+.

In Arrhenius's thoery, something that is a strong acid is a substance that completely ionizes in aqueous solution to give a hydronium ion, H3O+, and an anion. An anion is a negatively charged ion. An example of a strong acid is perchloric acid:

HClO4(aq) + H2O(l) -> H3O+(aq) + ClO4-(aq)

What is going on above is that we have perchloric acid in an aqueous solution. This perchloric acid ionizes entirely and results in an hydronium ion and a perchlorate anion. Some other examples of strong acids would be: HI, HBr, HCl, HNO3, and H2SO4.

Now on to bases...A strong base is something that completely ionizes in aqueous solution to give a hydroxide ion and a cation. A cation is a

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positively charged ion. Strong bases are most of the hydroxides of Group IA elements and Group IIA elements including LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2 and Ba(OH)2.

Many of the acids and bases that we encounter in our everyday lives are not strong acids, they are considered weak. Weak acids do not completely ionize in solution, but exist in equilibrium. Let's look at the reaction for acetic acid:

Bronsted-Lowery Concept of Acids and Bases

Helpful link: Visit the link below to view a video on the Bronsted-Lowery Concept.

http://www.backpack.tv/video/chemistry/washingtonville-high-school/mark-rosengarten/chemistry-tutorial-114a-bronsted-lowry

The Bronsted-Lowery concept of acids and bases is that acid-base reactions can be seen as proton-transfer reactions. This results in acids and bases being able to be defined in terms of this proton (H+) transfer. According to the Bronsted-Lowery concept, acids donate a proton in a proton-transfer reaction. Bases accept the proton in a proton-transfer equation. As an example, let’s look at the reaction of hydrochloric acid with ammonia. When we write it as an ionic equation we get:

H3O+(aq) + Cl-(aq) + NH3(aq) -> H2O(l) + NH4+(aq) + Cl-(aq)

which reduces to:

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H3O+(aq) + NH3(aq) -> H2O(l) + NH4+(aq)

because there is two Cl-(aq) one each side. We now have the net ionic

equation after we cancel out the "spectator ions"(Cl-).

What happens in this reaction in aqueous solution is a proton transfer. According to the Bronsted-Lowery concept, acids donate a proton in a proton-transfer reactions. Bases accept the proton in a proton-transfer equation. As an example, lets look at the reaction of hydrochloric acid with ammonia shown above. What happens in this reaction in aqueous solution is that a proton is transferred from H3O+ to NH3. This results in H3O+ losing a (H+), resulting in H2O. The NH3 gains the transferred proton, resulting in NH4

+. We call H3O+ the proton donor, or acid. We call NH3 the proton acceptor, or base.

The Bronsted-Lowery concept defines something as either an acid or

base depending on its function in the acid-base (proton transfer)

reaction. Some things can act as either an acid or a base. These are

called amphiprotic species; they can either lose or gain a proton,

depending on the other reactant. An example of an amphiprotic species

would be HCO3-. In the presence of OH-, it acts as an acid. In the

presence of HF it acts as a base. Water is also amphiprotic, as are most

anions with ionizable hydrogens and certain solvents. Water as an

amphiprotic species is very important to the acid-base reactions.

In the Bronsted-Lowery concept we have found that:

1. A base is a species that accepts protons, while an acid is a species that donates protons.

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2. Acids and bases can be ions as well as molecular substances.

3. Some species can act as either acids or bases, depending on what the other reactant is.

Lewis Concept of Acids and Bases

Helpful link: Visit the links below to view videos on ph and poh.

http://www.backpack.tv/video/chemistry/university-utah/chuck-wight/acids-and-bases

http://www.khanacademy.org/video/introduction-to-ph--poh--and-pkw

The Lewis concept of acids is generalized to include reactions of acidic and basic oxides and many other reactions. A Lewis acid is something that can form a covalent bond by accepting an electron air from another species. A Lewis base is something that can form a covalent bond by donating an electron pair to something else. The Lewis and Bronsted-Lowery concepts are different ways of looking at the same chemical reactions. Here is a reaction in which an electron pair is transferred. The proton (H+) is electron pair acceptor, A Lewis acid. NH3 has a lone pair of electrons and is a Lewis Base.

The pH of a solution

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Whether or not an aqueous solution is neutral, acidic or basic depends on the hydrogen-ion concentration. We give the acidity of an aqueous solution in terms of the pH. pH is defined as the negative logarithm of the molar hydrogen-ion concentration. A pH of 7 means that a solution is neutral. A pH of below 7 means that a solution is acidic; a pH of above 7 means that a solution is basic.

pH = -log[H+]

For example, let's say that we have a glass of frosty orange juice. This orange juice has a hydrogen-ion concentration of 2.9 x 10-4 M. What is the orange juice's pH?

pH = -log[H+] = -log[2.9 x 10-4] =3.54

The pH of this solution is less than 7 so this orange juice is acidic.

We can also find pH by solving for the hydroxide-ion concentration of a solution. The measure of the hydroxide-ion concentration is called pOH.

pOH = -log[OH-]

Since we know that the pH scale goes from 0 to 14, we find that:

pH + pOH = 14

Let's say that we want to find the pH of an ammonia solution that has a hydroxide-ion concentration of 1.9 x 10-3 M. We start by finding the pOH.

pOH = -log[1.9 x 10-3] = 2.72

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Now we want to find the pH by subtracting:

pH = 14.00 - pOH = 14.00 - 2.72 = 11.28

Equilibrium with Acids and Bases

Helpful link: Visit this link to view a video on equilibrium with acids and bases.

http://www.khanacademy.org/video/acid-base-titration

http://www.khanacademy.org/video/strong-acid-titration

http://www.khanacademy.org/video/weak-acid-titration

Have you read the section about equilibrium yet? If you haven't this most likely won't make any sense to you. If you have, let’s join in on the fun of acid-base equilibrium.

Remember Kc from the equilibrium section? It's back, and more useful

than ever. Now to distinguish between the Kc of acids and bases we use

Ka and Kb. (a for acids and b for bases). The equilibrium that is

calculated in acids is usually the disassociation of the H+ ions and the

rest of the molecule. The weak acids and bases are the only ones that

have Ka's and Kb's because in the strong acids dissociation is very close to 100%.

Table of Ka's

Substance Formula Ka

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Acetic Acid HC2H3O2 1.7 x 10-5

Benzoic Acid HC7H5O2 6.3 x 10-5

Boric Acid H3BO3 5.9 x 10-10

Carbonic Acid H2CO3 4.3 x 10-7

HCO3

- 4.8 x 10-11

Cyanic Acid HCNO 3.5 x 10-4

Formic Acid HCNO2 1.7 x 10-4

Hydrocyanic Acid HCN 4.9 x 10-10

Hydrofluric Acid HF 6.8 x 10-4

Hydrogen Sulfate ion HSO4- 1.1 x 10-2

Hydrogen Sulfide H2S 8.9 x 10-8

HS- 1.2 x 10-13

Hypochlorous acid HClO 3.5 x 10-8

Nitrous Acid HNO2 4.5 x 10-4

Oxalic Acid H2C2O4 5.6 x 10-2

HC2O4

- 5.1 x 10-5

Phosphoric Acid H3PO4 6.9 x 10-3

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H2PO4

- 6.2 x 10-8

HPO4

-2 4.8 x 10-13

Phosphorous Acid H2PHO3 1.6 x 10-2

HPHO3

- 7 x 10-7

Propionic Acid HC3H5O2 1.3 x 10-5

Pyruvic Acid HC3H3O3 1.4 x 10-4

Sulfurous Acid H2SO3 1.3 x 10-2

HSO3

- 6.3 x 10-8

The base Kb's are as follows:

Substance Formula Kb

Ammonia NH3 1.8 x 10-5

Aniline C6H5NH2 4.2 x 10-10

Dimethylamine (CH3)3NH 5.1 x 10-4

Ethylamine C2H5NH2 4.7 x 10-4

Hydrazine N2H4 1.7 x 10-6

Hydroxylamine NH2OH 1.1 x 10-8

Methylamine CH3NH2 4.4 x 10-4

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Pyridine C5H5N 1.4 x 10-9

Urea NH2CONH2 1.5 x 10-14

Examples

Weak Acid Example:

Calculate the pH of a 0.100 M solution of HClO

HClO <--> H+ + ClO-

.100 - x

x

x

Ka = __x2__

.100 - x

The x in the denominator can be dropped because Ka/M is less than

10-3. If Ka/M is greater than 10-3

you have to use the quadratic formula to solve the equation.

Therefore:

3.5 x 10-8 = __x2__

.100

3.5 x 10-9 = x2

x = 5.9 x 10-5

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[H+] = 5.9 x 10-5 M [ClO-] = 5.9 x 10-5 M [HClO] = .100 M - 5.9 x 10-5 ~= .100 M

pH = -log(5.9 x 10-5) = 4.2

Weak Base Example:

What is the concentration of OH- of a .20 molar solution of aniline?

C6H5NH2 <--> C6H5NH3+ + OH-

.20 - x

x

x

Kb = __x2__

.20 - x

The x in the denominator can be dropped because Kb/M is less than 10-

3. If Kb/M is greater than 10-3 you have to use the quadratic formula to solve the equation.

Therefore:

Kb = __x2__

.20

x2 = 8.4 x 10-11

x = 9.2 x 10-6

[OH-] = 9.2 x 10-6

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Lesson 13 Review

There is additional review information to assist you in complete this review. The review questions are in bold print.

1. Which of the following is a strong acid?

a) HI b) HF c) H3PO4

d) H2CO3 e) HClO2

2. Which of the following is a weak acid?

a) HI

b) HCl c) H3PO4 d) HNO3

e) HClO4

3. Bases turn litmus

a) Orange b) Blue c) Pink d) Red

4. A Lewis ______ is a species that can form a covalent bond by accepting an electron pair

a) Acid b) Base

5. Which of the following can be shown to be both a Bronsted acid and a Bronsted base?

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a) O-2

b) HSO4-1

c) PO4-3

d) CN-1

Acid or Base? A quick review….

Acids

Acids are compounds that taste sour, conduct electricity, and react with metals to

produce hydrogen gas. Acids contain one or more hydrogen ions. An example of a

weak acid is acetic acid (vinegar). Sulfuric acid, hydrochloric acid, and nitric acid

are all strong acids.

6. Conduct internet research to identify 3 additional acids.

Bases

Bases taste bitter, feel slippery, dissolve oils and fats, and contain one or more

hydroxide (OH-) ions. Magnesium hydroxide and calcium hydroxide are weak

bases. Sodium hydroxide is a very strong base.

7. Conduct internet research to identify 3 additional bases.

Neutral Substances

If something is not an acid or a base, it is neutral. Pure water, for example,

is a neutral substance.

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8. Conduct internet research to identify three additional neutral

substances.

pH

A scale called the pH scale is used to indicate the strength of an acid or a base.

On the pH scale, substances with a pH from 0 to less than 7 are acidic. Those

with a pH greater than 7 up to 14 are basic. Those with a pH of 7 are neutral.

Indicators

A substance's pH can be measured precisely using a pH meter. Approximate pH

can be determined using a chemical indicator. Indicators change color in the

presence of acids or bases. Bromothymol blue, phenol red, phenophthalein, and

litmus paper are examples of chemical indicators. Litmus is a common indicator

made from lichen plants. In acids, blue litmus paper turns red. In bases, red litmus

paper turns blue.

9. Write A if the substance is an acid. Write B if it is a base.

a) Human blood has a pH of 7.4.

b) A soft drink has a pH of 3 .

c) A tomato has a pH of 4.

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ACIDIC NEUTRAL BASIC

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d) Milk has a pH of 6.5.

e) Urine has a pH of 6.0.

10. Use the work bank to match each acid or base with the answer that tells

where it can be found.

Deodorant soft drinks oranges vinegar

drain cleaner stomach acid car battery

a) Ascorbic acid helps build strong teeth and bones.

b) Carbonic acid adds fizz to beverages.

c) Acetic acid has a sour taste and a sharp smell.

d) Hydrochloric acid forms naturally in humans, and it aids in

digestion.

e) Sodium hydroxide can break down grease and other

waste.

f) Sulfuric acid is a strong acid that is highly corrosive.

Submit your answers in the text box.